A switching power source is required to satisfy a low cost, miniaturization, high efficiency, a low voltage and a large current. Accordingly, as a switching element, an N-channel type power MOSFET (hereinafter abbreviated as NMOS) which is manufactured at a low cost and exhibits the low ON-resistance (low Ron) and the low Qgd (low gate charge quantity) is used in many cases. FIG. 13 shows the constitution of a step-down type switching power source which is studied prior to the present invention. In using the NMOS in a high potential-side switching element M1, a booster circuit which is referred to as “bootstrap” and a level shifting circuit shown in the drawing are necessary. In the booster circuit shown in FIG. 13, a voltage (VDD-Vf) which is lower than a power source voltage VDD by an amount corresponding to a forward voltage Vf of a diode D4 is supplied for driving high-potential side switching element M1. That is, the voltage which is higher than a source (middle point LX) of the switching element M1 by the voltage (VDD-Vf) stored in the bootstrap CB is supplied to a gate of the switching element M1. To increase the above-mentioned voltage (VDD-Vf), a Schottky barrier diode which exhibits a low forward voltage Vf is used as a diode D4.
FIG. 14 shows operational waveforms of respective parts of the step-down-type switching power source shown in FIG. 13. In the step-down-type switching power source, a middle point LX between the high-potential side switching element M1 and the low-potential side switching element M2 is changed to an input voltage Vin and a ground potential VSS for every switching. The booster circuit charges a bootstrap capacity CB from a power source voltage VDD through a diode D4 during a period that a potential of the middle point LX assumes a ground potential VSS. Accordingly, both-end voltages of the bootstrap capacity CB assume the voltage (VDD-Vf) which falls from the power source voltage VDD by an amount corresponding to a forward voltage Vf of the diode D4. When the potential of the middle point LX assumes the input voltage Vin, the diode D4 prevents a backward flow to the power source voltage VDD and supplies electricity to a driving circuit of the high-potential side switching element M1 from the bootstrap capacity CB. A driving voltage Vgs of the high-potential side switching element M1 becomes (VDD-Vf).
On the other hand, here observed is a tendency that the power source voltage VDD of peripheral circuits including a control circuit is lowered. Accordingly, the lowering of an amount of a forward voltage Vf of the diode D4 is no more ignorable thus giving rise to a possibility that the high-potential side switching element M1 cannot obtain a sufficient driving voltage. When the driving voltage becomes insufficient, the switching element cannot exhibit original performances thereof thus bringing about an increase of loss or the like. Accordingly, an example of a switching power source which constitutes a bootstrap circuit using a junction-type FET (hereinafter abbreviated as JFET) and incorporates an IC therein, there has been known a switching power source disclosed in Japanese Patent Laid-open Heill (1999)-501500. In this switching power source, the bootstrap capacitor is charged through the JFET. Patent document 1: Japanese Patent Laid-open Heill (1999)-501500